The present invention relates to improvements in a detection sensor for flammable gases and oxygen gas in flammable gases. More particularly, the present invention relates to the sensor for such uses as in securing and ensuring the safety of a variety of production equipment and facilities and detection of hydrogen gas in pure water for manufacturing semiconductors and hydrogen gas in gases for manufacturing semiconductors.
Among the detection sensors for flammable gases that have been used widely are the contact catalytic reaction type (or the contact combustion type) gas detection sensor, semiconductor type gas detection sensor and thermal conductivity type gas detection sensor. Of those types, the contact catalytic reaction type gas detection sensor has found wide uses for detection of such gases as hydrogen gas because of its long service life and reliability.
FIG. 10 shows a partially broken way view of an example of the sensor element A of the prior art contact catalytic reaction type gas detection sensor. This sensor element A includes a coil B of platinum wire some 20 xcexcm in diameter with which a mixture of a binder and alumina or silica alumina to be a catalyst support C is sintered, with a catalyst D like platinum supported therein.
The aforesaid sensor element A is incorporated in a bridge circuit for detection of gas concentration. That is, a bridge circuit is formed, as shown in FIG. 11, with the sensor element A and a temperature compensation element Ao made by sintering an inert substance. A specific voltage is applied to the sensor element A to raise before hand the temperature to not lower than some 250xc2x0 C. If a flammable gas like hydrogen gas comes in contact with the preheated sensor element A, the gas will undergo a contact catalytic reaction by the catalytic action of catalyst D and the sensor element A will be heated. That increases the electrical resistance of the sensor element A to break the equilibrium in the bridge circuit and to cause an electric potential difference. As a result, an indicator E turns. The extent of the turn of the indicator shows the heating value of the sensor element A, that is, the concentration of the flammable gas within the detection gas or subject gas under test.
The sensor element A as shown in FIG. 10 has such advantages as (a) high selectivity for flammable gases, (b) hardly influenced by the co-existing H2O and (c) suitable for measurement of gas concentration close to the lower limit of explosion (in the case of hydrogen gas, 1 to 4%).
However, the problem with the sensor element A as shown in FIG. 10 is that the temperature of the sensor element A has to be maintained at not lower than 250xc2x0 C. and that the working temperature further rises in detection of the flammable gas concentration. That could ignite the flammable gas. To ensure the safety, the sensor element A has to be made explosion-free by covering the sensor element A with such as a wire netting with a mesh of some 200 or sintered metal. In other words, the sensor element A as shown in FIG. 10 has a serious safety problem.
It is also noted that this type of sensor element A is so formed that catalyst D is supported within the catalyst support C as mentioned, and has a basic problem about the stability of catalytic activity. Especially, the effects on the catalyst in the sinter material by the burning of the flammable gas and the effects on the catalytic activity of carbon coming from incomplete combustion of the flammable gas haven not been elucidated well yet. Few studies are reported in which this type of the sensor element A is used for detection of a flammable gas which is present in small quantities in highly concentrated H2O or O2.
Another problem is that it is difficult to clean the inside of the catalyst support C of this kind of the sensor element A. For this reason, the sensor element A can not be used in the semiconductor manufacturing process where a high degree of cleanliness is required.
As set forth above, the application of the contact catalytic reaction type flammable gas detection sensor A for detection of a flammable gas present in small amounts in highly concentrated H2O or O2 presents problems with regard to reliability and the like. It is also the case with the use of the semiconductor type flammable gas detection sensor and the thermal conductivity type flammable gas detection sensor. Furthermore, while it is possible to use this kind of the sensor element A as sensor for detection of oxygen in the flammable gases in principle, it has not been put to practical use because of the aforesaid problems like reliability, and few application studies have been reported.
In other words, the prior art flammable gas detection sensor A of the contact catalytic reaction type drops substantially with lapse of time in catalytic activity, that is, H2 gas detection sensitivity. For reasons of poor reliability, the sensor element A can hardly be applied for such uses as detection of the concentration of unreacted hydrogen gas in the moisture take-out line of the moisture generating reactor for semiconductor manufacturing facilities. The same is the case with the aforesaid semiconductor type sensor for detection of hydrogen gas and the thermal conductivity type sensor for detection of hydrogen gas, which has been confirmed in experiments.
In the moisture generating reactor for semiconductor manufacturing facilities, it can happen that moisture is generated with excessive supply of hydrogen gas. In such a case, it is necessary to detect the concentration of unreacted oxygen gas in the generated moisture containing hydrogen in the moisture take-out line. The prior art sensor element A can not be used in such cases.
Meanwhile, the applicants of the present application developed a flammable gas detector as shown in FIG. 12 that solved the problems with the prior art contact catalytic reaction type sensor element A for detection of flammable gases and disclosed the same in unexamined Japanese patent application No. 9-186383.
This flammable gas detector is composed of a flammable gas detection sensor 20 and a detector unit 30. The flammable gas detection sensor 20 is formed of a first detection sensor 21 provided with a platinum coating catalyst, a second detection sensor 22 to detect the temperature of the detection gas (the gas to be detected) or subject gas under test and a sensor holder 23.
The detector unit 30 includes a first temperature detector 31 to detect the temperature signal from the first detection sensor 21, a second temperature detector 32 to detect the temperature signal from the second detection sensor 22, a first temperature display 33 and second temperature display 34 to display the temperatures detected by the aforesaid two temperature detectors respectively, a temperature difference detector 35 to detect the difference between the detected temperatures and a temperature difference display 36 to display the temperature difference from the temperature difference detector 35.
The flammable gas detection sensor 20 is placed in a T-shaped branch pipe 39 with the sensor holder 23 fitted in air-tight and with the two sensor elements 21, 22 held in a gas feeder pipe 37 as shown in FIG. 13. The T-shaped branch pipe 39 is provided with explosion proof metal meshes 38 in gas feeder pipe 37.
The flammable gas detector shown in FIG. 12 and FIG. 13 is excellent in responsiveness and gas concentration detection accuracy, and can correct the detected value without difficulty when the flow rate of the detection gas changes. Another practical advantage is that the change with lapse of years in detection sensitivity is relatively small.
But this flammable gas detector has a number of problems yet to be solved. Among the problems requiring urgent solution are contamination of the high-purity gas flowing through the pipe, the reliability of detection precision and safety.
To be specific, the first detection sensor 21 and the second detection sensor 22 to be placed in the high-purity gas are thermocouples, and the outer surface of the first detection sensor 21 to be heated by the contact catalytic reaction of flammable gas is coated with a platinum catalyst film via a barrier coat like TiN.
However, the adhesion strength between the metal, for example, chromel-alumel, forming the thermocouple and the barrier coat like TiN undergoes a change relatively fast with lapse of years. As a result, the platinum coat on the first detection sensor 21 could fall off, contaminating the high-purity gas. In other cases, partial peeling off of the platinum coat could reduce the catalytic reactivity.
The detection sensors may be formed of platinum. An example is a gas detection sensor that has the thermocouple of the first detection sensor 21 made of noble metals like platinum and rhodium and that has the second detection sensor 22xe2x80x94to detect the temperature of the fluidxe2x80x94also made of platinum and rhodium with the outer surface coated with a barrier coat like TiN. In this case, there is no fear that the barrier coat will come off with lapse of years.
However, the thermocouple of noble metals would be relatively expensive and present problems with mechanical strength and machining, which would make it difficult to put it to practical use.
The present invention address the following problems with the gas detection sensor of the type shown in FIG. 12: (a) the thermocouple made of base metals is liable to deteriorate in adhesion between the platinum coat and the thermocouple forming material with passage of years, with peeled off platinum coat contaminating the high-purity gas and reducing catalytic reactivity, (b) the thermocouple made of noble metals is expensive to manufacture, and (c), in the latter case, difficulty in machining and relatively low mechanical strength, making it difficult to reduce manufacturing costs.
It is accordingly an object of this invention to provide a gas detection sensor that is free from contaminating the high-purity gas and from changing with passage of time in detection precision, yet is excellent in safety and relatively inexpensive to manufacture.
The inventors have been engaged in developing reactors for generating moisture for many years, and in the course of the research and development they have succeeded in stabilizing the platinum coat formed on the inside wall of the reactor made of stainless steel, that is, preventing the catalytic performance from changing with time.
Then, the inventors have noticed that it is possible to build a flammable gas sensor with little deterioration in catalytic performance and with high reliability and safety at low costs if the technique for forming a platinum catalytic layer in the reactor for generating moisture is applied to a flammable gas detector.
On the basis of that finding, the inventors, thinking that the change in output caused by a change in the temperature of the thermocouple should be a factor in detecting the flammable gas concentrations, tested various characteristics of thermocouples of different types and studied the results in detail.
The present invention was made through such a process. The invention comprises a gas detection sensor wherein flammable gas detection signals are issued by the heating of the sensor caused by the contact catalytic reaction with the flammable gas. The gas detection sensor includes a first detection sensor having a diaphragm with a platinum coat on the gas contact surface coming in contact with the flowing detection gas, and a thermocouple having the one ends of two metal pieces of different kinds placed close to each other and fixed on the side of the diaphragm not coming in contact with gasxe2x80x94the reverse of the side that comes in contact with the detection gas and which is heated by the contact catalytic reaction of flammable gasxe2x80x94; and a second detection sensor which includes a diaphragm coming in contact with the flowing detection gas and a thermocouple having the one ends of two metal pieces of different kinds placed close to each other and fixed on the side of the diaphragm not coming in contact with gasxe2x80x94the reverse of the side that comes in contact with the detection gasxe2x80x94and which detects the temperature of the flowing detection gas.
An object of the invention is to provide a gas detection sensor wherein detection signals for oxygen gas in the flammable detection gas are issued by the heating of the sensor owing to contact catalytic reaction with the flammable gas. This gas detection sensor comprises: a first detection sensor which includes a diaphragm having a platinum coat on the gas contact surface coming in contact with the flowing detection gas and a thermocouple having the one ends of two metal pieces of different kinds placed close to each other and fixed on the side of the diaphragm not coming in contact with gasxe2x80x94the reverse of the side that comes in contact with the detection gasxe2x80x94and which is heated by the contact catalytic reaction of flammable gas; and a second detection sensor which includes a diaphragm coming in contact with the flowing detection gas and a thermocouple having the one ends of two metal pieces of different kinds placed close to each other and fixed on the side of the diaphragm not coming in contact with gasxe2x80x94the reverse of the side that comes in contact with the detection gasxe2x80x94and which detects the temperature of the flowing detection gas.
A further object of the invention is to provide a sensor as described above wherein the diaphragms in the first detection sensor and the second detection sensor are made of stainless steel and wherein a barrier coat is formed on the sides of the two diaphragms that come in contact with the gas.
According to the invention, the barrier coat is formed of an oxide or nitride and/or each thermocouple may be made of chromel-alumel.
A further object of the invention is to provide a sensor as described above wherein the diaphragms of the first detection sensor and the second detection sensor are fit into the detection sensor insertion ports of the stainless steel sensor block, with the gas-contact surface of the diaphragm facing the gas passage and with the inserting ports sealed airtight with the respective diaphragms. The stainless steel sensor block is each provided with an inlet and an outlet for detection gas, a gas passage through which the inlet communicates with the outlet and the first detection sensor inserting port and the second detection sensor inserting port which communicate with the gas passage.